Recently there has been much interest in the preparation of so-called "structural RIM" parts. Typically such parts are made by laying a glass fiber mat in a mold and introducing polyurethane or polyurea reactants into the mold via the reaction injection molding ("RIM") process. A particularly preferred RIM formulation is described in U.S. Pat. No. 4,792,576. Previously, the molds used to produce such parts utilized a metal-to-metal seal to prevent loss of materials during the process. However, even with good mold clamping pressures, the relatively low viscosity reaction components flowed between the seal giving the flash that is typical to RIM molding. A discussion of the flash phenomenon can be found in the book entitled "Reaction Injection Molding", edited by W.E. Becker, Van Nostrand Reinhold Company, 1979, pages 81, 90, and 234-235.
In addition, in those cases where the glass mat had not been cut to the exact shape of the mold, the reaction components flow preferentially along the edge of the part, rather than through the glass mat. This phenomenon leads to the formation of voids in the final part and to improperly filled molds.